A knocking sensor, which detects the knocking of an internal combustion engine of an automobile or the like, is known and the delay control of an ignition timing of a spark plug is performed according to the detection of the knocking sensor.
A knocking sensor, which has a structure shown in FIGS. 5 and 6, is known as the above-mentioned knocking sensor (Patent Documents 1 and 2). The knocking sensor 100 is a so-called center hole-type non-resonant knocking sensor that has a mounting hole 120b, through which the knocking sensor is mounted on a cylinder block of an internal combustion engine, at the central portion thereof.
As shown in an exploded perspective view of FIG. 6, the knocking sensor 100 includes a metal shell 120 that includes a cylindrical portion 121 and a flange portion 122 positioned at the lower end of the cylindrical portion 121. An annular lower insulating plate 130, a lower electrode plate 140, a piezoelectric element 150, an upper electrode plate 160, an upper insulating plate 135, a weight 170, and a disc spring 180 are fitted to the outer periphery of the cylindrical portion 121 in this order from the flange portion 122. Meanwhile, upper and lower terminals 141 and 161, which are used to take a voltage, are formed on the outsides of the lower and upper electrodes 140 and 160, respectively, so as to extend in the shape of a piece in the radial direction. A male screw portion 121b is formed on the upper outer peripheral surface of the cylindrical portion 121. Meanwhile, a female screw portion 185b is formed on the inner surface of a nut 185. Further, the nut 185 is threadably engaged with the male screw portion 121b. Accordingly, a laminated body, which reaches the disc spring 180 from the lower insulating plate 130, is interposed between the flange portion 122 and the nut 185, so that a sensor body 190 is formed (see FIG. 5). Furthermore, the sensor body 190 is covered with a resin molding 110 as shown in FIG. 5, so that the knocking sensor 100 is formed.
When being used, the knocking sensor 100 having this structure is mounted so that the lower surface of the flange portion 122 of the metal shell 120 comes into contact with the cylinder block. Accordingly, the metal shell 120 is electrically connected to (comes into electrical contact with) the cylinder block. As a result, the weight 170, which is electrically connected to the metal shell 120, is also electrically connected to (also comes into electrical contact with) the cylinder block. Accordingly, the lower insulating plate 130 that electrically insulates the flange portion 122 of the metal shell 120 from the lower electrode plate 140 and the upper insulating plate 135 that electrically insulates the upper electrode plate 160 from the weight 170 are used in the knocking sensor 100 as components of the sensor body 190 so that the insulation between the piezoelectric element 150 and the metal shell 120 and the weight 170 is secured. Meanwhile, a cylindrical insulating sleeve 131 is fitted to the outer peripheral surface of the cylindrical portion 121, so that the lower electrode plate 140, the piezoelectric element 150, and the upper electrode plate 160 are prevented from being electrically connected to the cylindrical portion 121.
[Patent Document 1] JP-A-2003-322580
[Patent Document 2] JP-A-2008-144677